58 J. S. GARDNER ON THE CRETxlCEOTTS DENTALIID^. 



Cadulus are nearly of the same proportions as those of the full- 

 grown animal, but differ in being transparent, whilst what I suppose 

 to be the older shells are opaque and glossy. If they increased in 

 layers, in the manner common to most Mollusca, half-grown in- 

 dividuals would be funnel-shaped, f-grown would show the 

 broad end closing in, which closing would continue until the full- 

 grown shape was reached. None of these intermediate forms have ever 

 been found ; and the question naturally presents itself whether the 

 shell does not begin to be formed over the fully grown animal, first 

 as a membrane, and gradually thickening until it becomes opaque. 

 This idea receives some confirmation from the fact that no shell of 

 any species of Cadulus shows traces of rings, but all are, on the 

 contrary, smooth. 



The examination of this family, the Dentaliidse, in its range 

 through Cretaceous rocks, shows that the forms which these tubular 

 shells have assumed have been no less persistent than those of the 

 patelliform shells. The smooth forms of Dentalium, the Entalis, 

 Siphodentalium, and Cadulus have remained almost unchanged 

 from far back in the Cretaceous period, through the Eocene, and 

 have persisted until now. The striated forms with thickened shells 

 also still exist in many recent species ; but the large alate striated 

 forms seem to have developed and become extinct in but a portion 

 of Cretaceous time. 



Genus Dentalitjm. 

 Group A. 

 Dentalitjm dectjssatum, J. Sby. Gault. (Plate III. figs. 1-12.) 



Shell cylindrical, thick, moderately curved, gradually tapering ; 

 apex obtuse, entire ; internally smooth and polished, without groove ; 

 internal cast gradually tapering, with the impress of the muscular 

 attachment. Scidpture : 12 unequal, rounded, longitudinal ribs ex- 

 tend from the apex to about -J the length of the shell in full- 

 grown specimens ; at about ^ of an inch from the apex other 

 ribs are inserted, and again others rise, until, near the point of dis- 

 appearance, 50 may be counted ; the ribs which continue from near 

 the apex are the more strongly marked throughout ; upon an average 

 every third rib is more prominent until near the point of disap- 

 pearance, where they become equal : the ribs are frequently marked 

 by lines showing interruptions of growth and by slight swellings : 

 they are crossed by strong, irregularly spaced, rounded striae, some- 

 times close together, sometimes with an interval of 3 times their own 

 width ; these striae become most closely set towards the aperture ; 

 slight swellings are visible at the intersection ; the whole shell is 

 covered with fine longitudinal striae. L. 3*25 in. B. 0-4125 in. 



Sowerby described this form in the ' Mineral Conchology,' vol. i. 

 p. 101, tab. 70. f. 5, from a very distinctly striated fragment. In 

 Mantell's ' Geology of Sussex,' he has named a smaller specimen D. 

 striatum (fig. iv. pi. 1). It is D. bicostale of De Ryckholt, who 

 also provides a name, D. Geinitzianum, for the specimen of the same 

 shell figured in Mantell. D. rhodani, D. cidaris, D. alternans, D, 

 Nysti are also closely allied, if not identical forms. 



